Determinism

Simplified sketch of the determinist approach to hazard

Deterministic seismic hazard is the first method
to go beyond observations of effects and look at the causes of seismicity.
The object of the deterministic method is to answer the question,
"what is the worst that could happen in an earthquake?". If you design
for the worst thing possible, then you are bound to be safe. The stages
in the method are:

Find the nearest active fault

Calculate the largest earthquake that could happen on this fault

Assume the largest earthquake happens at the closest point to
your site

Calculate what the ground motion will be

The advantages of this method are (a) that it is
relatively easily to do, and (b) it gives you a conservative answer
("conservative" in this context means a value that maximises safety).
The disadvantages are (a) often you don't know where the active faults
are (several of the largest Californian earthquakes of recent years
have occurred on faults that previously were unknown) and (b) it's
easy to come up with grossly over-conservative hazard values as the
largest earthquake possible on a fault may be about as common as a
50-ton meteorite impact. Possible, but not likely.

Recently an attempt has been made to improve the
deterministic method. Firstly, by including source areas as well as
faults, one can overcome the problem of hidden faults that you don't
know about. Secondly, the over-conservatism is addressed by modelling
not the maximum possible earthquake, but the "expected" earthquake
in the lifetime of the facility you are designing. A phrase one sometimes
hears is the "negotiated earthquake" - that is, the event that can
be agreed, by everyone involved with a project, is "sufficient" for
design purposes. The danger here is that one may be going from over-conservatism
to under-conservatism in that no allowance is made for the possibility
that an earthquake may occur that is a bit larger than what you were
expecting. However, the simplicity of the method is attractive, and
it may give a sufficient approximation to the hazard for structures
where the consequences of failure are not too great.

Another case where a deterministic approach is appropriate
is where a very large earthquake is so likely that you may as well
just model it directly - you can't ignore it!

Note that deterministic analysis does not remove
all uncertainty from hazard. The maximum earthquake that can occur
on a fault is never known perfectly, it can only be estimated within
a likely range of values. And you can never be sure exactly how strong
will be the ground motion at a site even if you know the magnitude
and the distance.